One additional point to ranochigo's answer: If multiple buckets have sufficient funds to cover a transaction, then Electrum will preferentially select the bucket with the fewest inputs.
Furthermore I don't use the right click context menu "Spend from" to manually select the addresses
which should be used for the next transaction.
On a slight tangent from your question, you can also click on "View -> Show Coins" to display an additional "Coins" tab across the top of Electrum. This tab shows every unspent UTXO in your wallet, and which address they are stored on. If you have multiple inputs on a single address, then this makes it possible to not spend them all at once if you don't want to, unlike the "Spend from" option.